Method of making a casting mold



United States Patent Ofifice 3,339,621 Patented Sept. 5, 1967 3,339,621 METHOD OF MAKING A CASTING MOLD Ary Bolk, Wilhelmus Cornelis Petrus Maria Meerman, and Petrus Johannes Buysman, Emmasingel, Eindhoven, Netherlands, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Continuation of application Ser. No. 395,563, Sept. 10, 1964. This application Nov. 14, 1966, Ser. No. 594,256 Claims priority, application Netherlands, Sept. 11, 1963, 297,791 8 Claims. (Cl. 16426) ABSTRACT OF THE DISCLOSURE A method of making a mold for the casting of metal articles in which a pattern is first coated with a layer of a fine-grained refractory powder containing a binding agent to which a layer of coarser-grained refractory powder is applied while the first layer is externally wet. The so-coated pattern is then placed in a molding box, the solidified coating layer Wetted with a binder and the socoated pattern surrounded with a binder-free granular refractory powder which serves to support the so-coated pattern such that the majority of the binder-free refractory powder can be removed and reused.

This application is a continuation of application Ser. No. 395,563, filed Sept. 10, 1964, and now abandoned.

The invention relates to a method of making molds, particularly molds for casting metal articles.

In making molds to be used in casting comparatively large metal articles the surfaces of which have to satisfy exacting requirements, with the use of a known method for making such molds one is confronted with problems of different characters.

In the known technique of casting by means of sand molds the surface of the castings are not sufficiently smooth owing to the coarseness of the sand used in making the mold. In addition, a certain reaction of the molding sand with the hot metal frequently occurs.

Several proposals have been made for the purpose of obviating the said disadvantages. For example, a layer of a paste consisting of graphite or zirconium silicate has been applied to the inner wall of the completed mold. However, this gives rise to other diificulties, for example, the smoothness of the inner wall of the mold and the dimensions of the casting are adversely affected.

It is also known to make a mold by coating a pattern with a layer of hardening pattern sand consisting of cement sand impregnated with glue or oil, arranging the coated pattern in a molding box and filling up the box with normal molding sand which may contain loam. Even with large size molds the coating of pattern sand may be comparatively thin. This known method, however, does not provide molds enabling castings having very smooth surfaces to be directly made. To obtain smooth surfaces the castings must be subjected to complicated and expensive finishing treatments.

In making molds for use in the lost pattern method, it is known to coat the pattern with a layer (coating layer) by means of a suspension of a fine-grained refractory powder containing a binding agent. The layer, while still moist, is sprinkled with a coarser refractory powder, and the resulting layer allowed to solidify. The pattern provided with the solidified layer is then arranged in a mold and this mold is filled up with a granular refractory powder in order to support the thin first layer by means of a supporting mass. The last-mentioned powder may contain a binding agent.

This method suffers from various limitations. The use of the supporting mass containing no binder results in molds the inner surface of which is readily damaged. The use of a supporting mass containing a binder has a limitation in that when the mold need no longer be used, the solidified mass cannot readily be re-used for making molds, but first has to be broken up and ground.

The method in accordance with the invention does not suffer from the aforementioned limitations. This method readily and directly provides molds having highly smooth inner surfaces which are not likely to be damaged, and enables substantially the entire material of the supporting mass to be readily used repeatedly.

The invention relates to a method of making a mold, particularly a mold for casting metal articles, in which the pattern to the surface of which a release agent may be; if desired, applied, is coated with a coating layer of a finegrained refractory powder containing a binder. A refractory powder of coarser grain is applied to the surface of the said layer when it is still moist externally. The coating layer is solidified and the pattern surrounded by the coating layer is arranged in a molding box. The coated pattern is surrounded by a granular refractory powder acting as a supporting mass, and is characterized in that the solidified coating layer is wetted by a binding agent.

The method in accodance with the invention is of particular importance in the method of the releasing pattern, especially for making molds for large-size castings the surfaces of which have to satisfy comparatively stringent requirements.

A pattern may be made from materials known for this purpose, for example, metal, wood or synthetic material. If desired, the model may be coated with a thin layer of a release agent, for example, Vaseline or silicone grease.

for example, by immersing the pattern, if desired repeatedly, in an ethanolic suspension of a mentioned kind and a binding agent.

The refractory powder applied to the coating layer when the latter is still externally moist, for example, by sprinkling, may be aluminum silicate (particle size from 250 600,14).

The coating layer may be caused to solidifygenera1ly by gelation-by a known method. When an acid silicicacid sol is used as a binder solidification may be effected by the action of ammonia or by spraying the layer with a suitable buffer solution, for example, an aqueous solution containing 700 gms. of ammonium acetate per liter, or by heating.

The material used for the supporting mass may be normal molding sand. To provide the supporting mass the molding box containing the pattern covered by the coatinlg1 layer may be disposed, for example, on a vibrating ta e.

The binding agent used to wet the solid coating layer may be a substance the use of which as a binder is known in making molds, and especially one of the substances used in making molds for precision castings. A silicic-acid sol is preferably used. The term silicic-acid sol as used powder of the aforeherein includes a solution of polysilicates in which a greater or lesser amount of alkyl radicals, for example ethyl radicals, are present in the polymeric molecules, and a solution of polysilicic acids. An example of another binding agent is a solution of sodium silicate. Further suitable binders are albumins, such as gelatine, and synthetic substances, such as methyl cellulose. The binding agent is preferably used in the form of an aqueous solution or dispersion.

The binding agent by which the solid coating layer is Wetted must not solidify before or immediately after the provision of the supporting mass. Before the binder solidifies, material of the supporting mass in contact with the coating layer must be satisfactorily wetted so as to produce, after solidification of the binding agent, satisfactory bonding of the coating layer and the supporting mass.

It has been found that sufficient bonding between the coating layer and the supporting mass is obtained if the binding agent wets a layer of the supporting mass having a thickness of a few millimeters, for example, from 4 to 5 mm., before solidifying.

To promote the wetting of part of the supporting mass by the binding agent a surface-active substance may be added to the solution of the binding agent or to the binding agent.

The said surface-active substance may, dodecylnaphthyl sodium sulphonate.

As set forth hereinbefore, the solidification of the binding agent must be eifected after a suflicient portion of the supporting mass has been wetted. The transition to the solid state will frequently be effected by gelation. Gelation will usually have to be initiated by means of an agent. This agent may be present in or added to the material of the supporting mass. In an example of a technologically very attractive embodiment of the method in accordance with the invention an acid gellable solution of a binding agent is used in combination with an alkaline powder, for example a naturally alkaline molding sand. In this embodiment a highly suitable acid gellable solution of a binding agent is an acid silicic-acid sol which may, for example, be obtained by hydrolysis of organosilicates, for example tetra-ethyl silicate or condensed ethylsilicate, with the aid 'of hydrochloric acid or by acidifying a solution of sodium silicate (water glass).

for example, be

A particularly suitable substance for wetting the solidified coating layer is a silicic-acid sol in which the instant of commencement of gelation is determined by the addition of a buffer solution. The commencement of gelation may, for example, be set to 4 minutes after the addition of the buffer solution. This was achieved, for example, by adding 3 cc. of an aqueous buifer solution containing 600 gm. of ammonium acetate and 100 gm. of acetic acid per liter to 100 cc. of a silicic-acid sol produced by stirring a mixture of 100 cc. of polyethyl silicate, 75 cc. of ethanol,

18 cc. of water and 7 cc. of l N hydrochloric acid for 30 minutes.

What is claimed is:

1. In the method of making a mold for casting metal articles comprising the steps of coating a pattern with a layer of fine-grained refractory powder containing a binding agent, applying to said coating layer while externally wet a coarser grained refractory powder, solidifying said coating layer, placing the so-coated pattern in a molding box, wetting the solidified coating layer with a binder, and surrounding the so-coated pattern with a binder-free granular refractory powder serving to support the so-coated pattern such that the majority of said binder-free granular refractory powder may subsequently be removed from said molding box in its original binder-free condition.

2. A method as claimed in claim 1, in which the binder used to wet the coating layer contains a surface-active substance.

3. A method as claimed in claim 1, in which the material used for the supporting mass contains an agent by the action of which the binder used to wet the coating layer solidifies.

4. A method as claimed in claim 3, in which a silicicacid sol is used as the binder for wetting the coating layer and a buffer is used as the agent.

5. A method as claimed in claim 3, in which an acid binder is used to wet the coating layer and the material used for the supporting mass contains an alkaline substance as the agent.

6. A method as claimed in claim 3, in'which an acid silicic-acid sol is used as the binder for wetting the coating layer.

7. A method as claimed in claim 3, in which a naturally alkaline molding sand is used as the material for the supporting mass.

8. A method as claimed in claim 1, in which the binder for wetting the coating layer is an acid silicic-acid sol, to which a buffer solution containing ammonium acetate is added.

References Cited UNITED STATES PATENTS 2,806,269 9/1957 Dunlop 164--26 2,948,935 8/ 1960 Carter 164--26 FOREIGN PATENTS 823,970 11/ 1959 Great Britain. 1,339,758 9/1963 France. 1,143,609 2/ 1963 Germany.

I. SPENCER OVERHOLSER, Primary Examiner.

E. MAR, Assistant Examiner. 

1. IN THE METHOD OF MAKING A MOLD FOR CASTIN METAL ARTICLES, COMPRISING THE STEPS OF COATING A PATTERN WITH A LAYER OF FINE-GRAINED REFRACTORY POWDER CONTAINING A BINDING AGENT, APPLYING TO SAID COATING LAYER WHILE EXTERNALLY WET A COARSER GRAINED REFRACTORY POWDER, SOLIDIFYING SAID COATING LAYER, PLACING THE SO-COATED PATTERN IN A MOLDING BOX, WETTING THE SOLIDIFIED COATING LAYER WITH A BINDER, AND SURROUNDING THE SO-COATED PATTERN WITH A BINDER-FREE GRANULAR REFRACTORY POWDER SERVING TO SUPPORT THE SO-COATED PATTERN SUCH THAT THE MAJORITY OF SAID BINDER-FREE GRANULAR REFRACTORY POWDER MAY SUBSEQUENTLY BE REMOVED FROM SAID MOLDING BOX IN ITS ORIGINAL BINDER-FREE CONDITION. 